The active muscles can use the sugar (glucose) that is in the blood stream for energy. Some of this sugar is stored in the muscles. When these local stores are used up, extra sugar continues to flow from the liver into the blood stream, and if the muscles use this sugar faster than can be supplied by the blood, the blood sugar level will fall.

In a person with diabetes, or in diabetes treat by diet alone, the pancreas makes less insulin during exercise and a drop in blood sugar is avoided.

In the person with insulin-treated diabetes, the insulin coming from the injection site during the exercise stays the same or may even be absorbed more quickly particularly if the dose has been given in an exercising limb, and a drop in blood sugar may occur.